The present invention relates to differential drive mechanisms, particularly automotive differentials of limited slip type, and is concerned with that type of differential drive mechanism which comprises a cage, which is rotatable about an axis and represents the input, two coaxial output shafts which are rotatable with respect to the cage about the said axis, a coupling which is connected eccentrically to the two output shafts to transmit relative contra-rotational movement between them by connections which permit relative rotation of the coupling and the output shafts about an axis substantially parallel to the said axis and a restraint member which is coupled to the cage and to the coupling such that the coupling is rotatable with respect to the cage about an axis substantially perpendicular to the said axis and capable of reciprocating movement in a direction perpendicular to the said axis but prevented from movement in a direction parallel to the said axis, the eccentric connection of the coupling and the output shafts including a respective eccentric hole in the inner end of each output shaft in which the associated end of the coupling is received, the ends of the coupling having a part-spherical engagement surface, a slipper which affords a complementary part-spherical internal surface being received in each eccentric hole and each engagement surface being in engagement with a respective complementary internal surface. Such a drive mechanism is disclosed in EP-A-0611166.
The provision of these slippers is very important because if they were not present the part-spherical engagement surface at the ends of the coupling would directly engage the surface of the associated eccentric hole whereby the contact between these two surfaces would necessarily be a point contact. In use, the force transmitted between the coupling member and the inner ends of the output shafts is very substantial indeed, and if this were transmitted through a point contact it would produce very substantial contact loads. When the differential is differentiating, that is to say when the two output shafts rotate relative to one another, this would rapidly lead to abrasion and destruction of the ends of the coupling and the surface of the eccentric holes.